Biology of Mangroves
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One of the most unique and least understood environments found in nature is that of the mangrove. This ecosystem is found at the junction between land and sea. Author, Loren Eiseley (1971) wrote vividly about his encounter with a mangrove forest in the book The Night Country:
A world like that is not really natural. Parts of it are neither land nor sea and so everything is moving from one element to anotherÖNothing stays put where it began because everything is constantly climbing in, or climbing out, of its unstable environment.
The actual word ëmangroveí can be used in two ways. It can refer to an individual species of plant or it can indicate a group or forest of plants that contains many species (Figure 1). To determine what constitutes a mangrove species many aspects are considered including: (1) air temperature within a certain range, (2) mud substrate, (3) protection, (4) salt water, (5) tidal range, (6) ocean currents and (7) shallow shores. To clarify, each of these aspects will be considered in turn. The best mangrove development has been found to occur only when the average air temperature of the coldest month is higher than 20 degrees Celsius and where the seasonal range does not exceed ten degrees. Most extensive mangroves are associated with muddy soils along deltaic coasts, in lagoons and along estuarine shorelines. In order for mangroves to survive a protected coastline is essential as mangrove communities cannot develop where high levels of wave action prevent the establishment of seedlings. In terms of salt water, there is increasing evidence that most mangroves have their optimal growth in the presence of some additional sodium chloride. The tidal range plays an important role in mangrove development. The greater the tidal range, the greater the vertical range available for the community. Also for a given tidal range, steep shores tend to have narrower mangrove zones than do gently sloping ones. Another aspect of mangrove communities is ocean current. The currents are essential since they act to disperse the mangrove propagules and distribute them along the coasts. The need for a shallow shore is the last major aspect of mangrove forests. This is apparent when considering that seedlings cannot become anchored in deep water and that the mangrove requires a large proportion of its body to be above the water (Hutchings and Saenger, 1987).
Mangrove forests are established in various regions of the world and one of the most prominent regions is the continent of Australia.
The area occupied by mangrove vegetation on Australiaís coastline is estimated at 11,617 square km (Teas, 1983). Most of the mangrove communities are found in the northeastern portion of the continent in the state of Queensland. Here at least 33 mangrove species belonging to sixteen families of angiosperms have been recorded (Teas, 1983). The number of mangrove species differs quite substantially between the west and east coast of the continent (Figure 2). This distribution gradient is most likely associated with the difference in availability of the before mentioned seven conditions needed to support a mangrove community. More specifically, the concentration of mangrove species in northeastern Australia can be attributed to three main factors: (1) this region was the center of origin of mangroves, (2) the climatic regime of this area is similar to that under which mangrove vegetation first developed and (3) the Great Barrier Reef provides large areas of low-energy coastline suitable for mangrove development (Hutchings and Saenger, 1987).
Distribution of mangrove species is not only obvious among different regions of Australia, but is also seen as a pattern of zonation running parallel along the shoreline. Usually the seaward side contains Avicennia species that are best adapted to early colonization and a wide range of soil conditions. The most common of these species is the grey mangrove. Moving inland the next zone is characterized by firmer soils that contain high salt concentrations. The yellow mangrove from the genus Ceriops dominates this zone. The wetland zone is next with smaller salt concentrations and even firmer soils. Here littoral vegetation merges into rainforest (Hutchings and Saenger, 1987). This zonation pattern is evident along open coasts, but a study by Bunt and Williams (1981) found this to be far less common in associated riverine estuaries. Further study by Bunt showed that no such zonation uniformity could be found for riverine estuaries as had been discovered for open shore line mangrove communities. (Bunt, 1996).
In order to sustain a successful dominance in such a unique environment a plant must have evolved with some unique adaptations. This is true for all species of mangrove. They have various adaptations that allow them to survive in an ecosystem that discourages competition from other plants. Three of these adaptations will be discussed in detail.
The first major obstacle mangrove species have adapted to overcome is that of high salinity concentrations. There are species that utilize salt excreting glands such as Avicennia, Laguncularia, Aegialites, Aegiceras and species that lack such glands but have distinct water storage tissue. This is evident in the species of Rhizophora, Sonneratia, Lumnitzera (Chapman, 1975). There is variation in the ultrastructure of the salt secreting glands among the different species of mangroves. Also different compounds that help balance total leaf osmotic pressure can be found in differing species. For example, choline-O-sulphate has been reported in Avicennia and Aegialitis while choline-O-phosphate is present in Aegiceras. Another interesting difference among species is whether the salt glands are present when there is no threat of high salt levels. In Avicennia, salt glands are only formed under saline conditions. In contrast, Aegiceras are found with salt glands regardless to whether or not salt is present in the medium (Hutchings and Saenger, 1987).
Two more processes for dealing with high salinity levels are less understood, but are of great importance. One involves the root system and is a salt exclusion method that utilizes an ultra-filter at the root level that is highly successful. These species are able to exclude 80-90 percent of the salt from the sea water (Hutchings and Saenger, 1987). The last method of salt control is loss of leaves with their contained salt (Chapman, 1975).
Besides high salinity levels mangrove communities must overcome respiration obstacles when inundated with tidal waters. The water displaces air from the soil and thus reduces the oxygen supply available to the roots (Curran, Cole and Allaway, 1986). To overcome this dilemma mangrove species rely on specialized root systems such as pneumatophores (Figure 3). These root projections arise from the cable root system and extend upward into the air as small conical projections. Evidence that these root projections act to provide aeration for subterranean comes from the observation that those mangroves growing at lower tide levels and which are, consequently, more frequently inundated tend to possess the greatest numbers of above ground root systems. The mangroves rely on such root projections only when the soil is poor and has little aeration. A study by Gessner (1967) showed that for Avicennia germinans growing on well aerated soil, the removal of pneumatophores had little effect on the trees. But in the case of poor aerated soil the covering of the pneumatophores with water or soil resulted in mortality (Hutchings and Saenger, 1987). It has also been shown that the air within a pneumatophore has a composition more closely related to that of the external atmosphere than to that of the soil atmosphere (Chapman, 1975).
The study by Curran, Cole and Allaway (1986) focuses on the question of whether the supply of oxygen to the root system is a passive process involving only diffusion or if some ìpumpingî action is involved. Their results found that diffusion alone is sufficient to supply the oxygen needs for the roots of Avicennia marina. Furthermore, the aerenchyma within the pneumatophore acts as an oxygen storage to help the tree maintain aerobic conditions when inundated by tidal waters.
The third major obstacle that mangroves have adapted to is water loss and how to preserve a suitable water balance. The water is saline compared to the internal sap concentration of the mangrove so it must be taken up against an osmotic gradient. The amount of this water that can be desalinated is based upon the amount of metabolic energy the tree has available. It is easily understood how costly this process is and, therefore, how important it is for the tree to have some mechanism for water preservation. The features that help maintain water are referred to as xenomorphic features. These features include a thick-walled epidermis, thick waxy cuticle and sunken stomata (Hutchings and Saenger, 1987).
A study performed by Misra, Choudhury, Ghosh, and Dutta (1984) looks at a mangrove species in eastern India. Their study focuses on the adaptation of these mangrove trees to submersion by tidal water. They collected leaf samples from the exposed environment on the slopes where tidal water merely reached the root zone. They then collected samples from the same species on trees whose leaves were submerged in tidal water. The analysis performed on these samples included lipid extraction and separation of wax esters and hydrocarbons, determination of leaf variables and determination of chlorophyll contents of the leaves. Their results showed the loss of water from leaves of the exposed plants was two to four times greater than in leaves of the periodically submerged plants. Also the leaf area increased significantly in the submerged plants for all species studied. In addition they concluded that higher amounts of was esters and hydrocarbons were synthesized during submersion. This accumulation of wax esters and hydrocarbons may act to help block out excess water during submersion and, in contrast, when exposed to air this increased accumulation may help to retain moisture.
Mangrove communities not only inhabit individual mangrove plants, but also act as a home for many types of fauna. Although these ecosystems are not the primary habitat for terrestrial fauna, many terrestrial animals spend time within the confines of mangrove forests. This occurs for various reasons: (1) fewer predators or competitors present, (2) mangroves provide abundant food supply at critical times of the year, (3) the flora is comprised of species with succulent leaves and (4) the abundant detritus on the forest floor may be important for some insect species (Hutchings and Saenger, 1987).
Most terrestrial vertebrates are not restricted to mangroves, but act as visitors. Mammals are mostly represented by varying species of rats including Xeromys myoides, water-rats, house mice and tree-rats. Larger mammals such as bandicoots, possums and swamp wallabies also inhabit the mangrove communities in the Northern Territory and Queensland. Birds are the most endemic animals in mangroves. Of the more than two hundred species of birds found in Australian mangroves fourteen species are virtually restricted to mangroves, twelve species use the mangrove as primary habitat in at least part of their range, and sixty species utilize it regularly throughout the year. Within the tropical mangroves reptiles are quite common, but they are rarely seen in temperate forests. Most reptiles use the mangroves as peripheral habitats (Hutchings and Saenger, 1987).
Among the terrestrial invertebrates only insects and spiders utilize mangrove communities. Termites, mosquitoes and biting midges make up the most highly studied group of insects within mangroves due to their economic and medical importance. Beyond these terrestrial invertebrates others such as butterflies, moths, ants and spiders have been noted to inhabit mangroves (Hutchings and Saenger, 1987).
The marine fauna proves to be more successful over the freshwater fauna in mangrove forests. Marine vertebrates include fish and reptiles. One of the most popular reptiles is the saltwater crocodile, Crocodylus porosus. These reptiles come into the mangroves to feed during high tide. They mostly chose sesarmid crabs, prawns and mudskippers as juveniles and then move on to large mud crabs, birds and mammals as they grow larger. These crocodiles do not nest in mangroves, but instead on the banks where the river comes close to the adjacent floodplain (Hutchings and Saenger, 1987). Perhaps the most abundant inhabitants of the mangrove forests are the various fish species. Sampling within a small mangrove-lined estuary in Queensland yielded 112,481 fish from 128 species and 43 families. The dominant families for these species included Engraulidae, Ambassidae, Leiognathidae, Clupeidae and Atherinidae (Robertson and Duke, 1990).
The marine invertebrates comprise a large proportion of the mangrove inhabitants. The most evident marine invertebrates are different types of crabs. These crustaceans are represented by large numbers of species in several families. Other common invertebrates include bivalves, barnacles and polychaetes. While many of the shrimps and amphipods are restricted to lower levels of the mangrove shore, crabs tend to occur throughout the mangrove zone (Teas, 1983).
In order for these various species of organisms to live in the harsh conditions of mangrove forests many have evolved their own adaptations. One such adaptation is seen among three different types of invertebrates regarding their individual micro-habitats. The polychaete builds soft, collapsible tubes plastered to the fronds of the algae while the sipunculan lives in tight-fitting burrows at or near the surface of the root-mat and the snapping shrimp lives in rather large open burrows in the root-mat. Each of these micro-habitats coincides with how each of the organisms handle the stress of the environment. The shrimp must be able to react efficiently and rapidly to changes in temperature and salinity due to its open burrow home. The polychaete, in contrast, lives in an environment where the water movement can be greatly slowed down by the presence of algal-mat. This is a helpful barrier since this soft-bodied worm has little resistance to high temperature or high salinity (Ferraris, Fauchald and Kensley, 1994).
Other examples of adaptations to mangrove habitats come from vertebrates. Endemic bird species show the most adaptations to the mangrove habitat and most of these are concerned with feeding. Longer bills are found in the mangrove robin, white-breasted whistler, mangrove fantail, dusky gerygone, red-headed honeyeater and the mangrove gerygone. Perhaps this is to prevent the clogging of bristles around the mouth and muddying of the face while foraging for food on surface mud. Also the white-breasted whistler has a hooked beak for the cracking of crustacean shells. Another adaptation is the more rounded wing and tail of the mangrove robin as compared to other species of the same genus. This difference is believed to allow for greater maneuverability as the robin flies through the mangrove canopy (Hutchings and Saenger, 1987).
Another great example of physiological adaptations to the mangrove habitat is seen in the mudskipper (Figure 4). Mudskippers are fish related to gobies and are characterized by their fused pelvic fins. They are found in tropical mangroves and are well adapted to varying degrees of tidal levels from exposure to air to complete submersion. They have very mobile eyes that compensate for the lack of a neck. The eyes are set in turrets and are protected from drying out by secondary spectacles. Since the eyes are set high on top of the head their field of view is increased. The mudskipper also has accessory respiratory surfaces on its fins and in the nasal sac diverticula. It is not known whether these additional surfaces aid in respiration or if they are associated with salt regulation. Besides normal fishlike swimming the mudskipper has three other forms of locomotion due to its modifications in skeletal structure and musculature. The first is termed ëcrutchingí since the pectoral fins are used as crutches. The second is a type of skipping on land that is normally used as an escape reaction. The last type is skimming across the water in a series of bounds where each bound is preceded by a short burst of swimming.
Other adaptations of great importance to mangrove inhabitants are those concerned with the salinity of the environment. Lizards have a nasal gland that secretes brine into the nasal cavity from which it is sneezed. Crocodiles use a number of salt glands located on the tongue and sea turtles have salt glands that are modified into tear glands associated with the eye. Many other salt secreting examples are found among reptiles such as sea snakes, colubrid snakes and goannas (Hutchings and Saenger, 1987).
The mangrove ecosystem has many unique characteristics associated with it that gives it extreme value. This ecosystem is one of great importance to many organisms surviving todayóincluding humans. Not only do mangroves provide nesting and breeding sites for many animals, but they also play a large role in maintaining the natural balance of the food chain. Mangroves provide great amounts of nutrients that feed the smallest of organisms, bacteria. Researchers have found ten billion bacteria living in one teaspoon of mangrove mud from a Queensland forest (Hutchings and Saenger, 1987). These bacteria along with fungi convert relatively indigestible lignin and cellulose from the plant tissue into a protein source that in turn can be digested by higher organisms. This organic matter does not only benefit the immediate higher organisms in a mangrove forest, but is also transported to benefit organisms in surrounding areas. For example, studies of mangroves at the northern end of Hichenbrook Island have shown they export greater than 12,500 tons of litter each year to the Great Barrier Reef waters (Hutchings and Saenger, 1987).
This leaf litter processing is not accomplished by bacteria and fungi alone, but is first tackled by various invertebrates. The most active leaf-shredders appear to be differing species of crabs. A study by Camilleri (1992) looks closely at the leaf processing abilities of invertebrates. He concludes, ìÖtwelve species of leaf-shredders make particulate organic matter originating from mangroves available for consumption by at least 38 other species of invertebrates.î This conclusion shows how many species of invertebrates are dependent upon the mangrove leaves which fall from the tree as a food source (Camilleri, 1992).
Mangrove forests also play an important economic role when looking at the fishing industry. The coastal fish and prawn populations that rely on the mangrove ecosystem help to support fisheries worth A$700 million a year. Research in Australia has shown the mangrove habitat as an important feeding and shelter site for juvenile banana prawns. This reliance on mangroves by a variety of fish has also been found to be true in Malaysian mangrove forests (Robertson and Duke, 1990).
The uniqueness of mangrove forests not only makes them valuable, but also very fragile. Although Australian mangrove forests have a fairly bright future due to the countryís economic and population characteristics, there is still great concern for their preservation around the world. For many years mangrove forests have been abused as wastelands. They have been used as sanitary landfills and converted into oxidation ponds for the tertiary treatment of sewage effluent. The land is also threatened by the charcoal industry, tourism development and coastal pollution, including oil spills. The latter is extremely damaging to mangroves because of their structure. Since waves and currents on the shoreline transport floating oil, low wave energy ecosystems like mangroves are accumulation sites. Also the inaccessibility of mangroves make oil removal extremely difficult. In addition the burrowing activities of crustaceans lead to high levels of oil contamination not only on the surface but also deep into the sediment (Teas, 1975). And although prawn aquaculture is very profitable, it is a major source of destruction to the mangrove forests. The prawn farmers clear large areas of mangrove forest for the construction of large artificial ponds. The worst part to this massive removal of such a unique forest to make room for prawn ponds is that within a few years the prawn farm is forced to shut down because of insurmountable pollution problems that plague the ponds. This clearing of the natural mangrove forests leads to various related problems such as: a loss of invaluable wildlife habitat, a serious decline in the worldís tropical coastal fisheries and coastal destabilization in the form of heavy erosion and siltation resulting in loss of both sea grass and coral reefs (A. Quarto, 1992).
Those working to save the mangrove ecosystem realize the only way to accomplish this large task is through continued research, careful management and advertising the problem to citizens in order to gain support which in turn could help persuade the political leaders to look more closely at the situation. In 1982, Odum states:
The situation in Australia today is that mangrove ecosystems are not recognized as a valuable national asset by most decisionmakers nor by the community generally. Mangroves are not managed by any single authority as a national resource as are terrestrial forests, national parks, mining and fishing. In the absence of a coherent attitude towards this resource, management decisions in relation to mangroves are taken in a piecemeal fashion. The development area is seen in isolation and the regional context or the catchment area of the particular site is conveniently ignored. The result is a constant gnawing away of the resource without taking into account the full implications of the impact it makes (Hutchings and Saenger, 1987).
The awareness of this destruction has grown and more organizations are working to improve the situation. Hopefully in time the vast importance of mangroves will be realized by the world and management plans can be created to preserve what remains of such a valuable natural resource.